The present invention relates to a methane fermentation reactor or, more particularly, to a two-phase type methane fermentation reactor for generation of methane gas as a useful fuel gas by subjecting an organic waste material such as livestock excrements to a fermentation treatment therein.
It is an established industrial process to produce a methane-based fuel gas by methane fermentation of a variety of organic waste materials including livestock excrements. Several types of methane fermentation reactors for the fermentation treatment of livestock excrements have been developed and actually employed including variable-volume gas-holder type reactors having a built-in gas holder with an up-and-down movable canopy to change the capacity of the gas holder depending on the holding volume of the methane gas, fixed gas holder type reactors in which the gas holder is fixed to the upper part of the fermentation tank and two-phase type reactors having a sewage digestion tank, conventionally used for an anaerobic treatment in sewage disposal plants, an acid fermentation tank and a methane fermentation tank.
EP-A-0 335 825 discloses a bioreactor for two-phase methane fermentation which comprises a tank that has two compartments (a first compartment for acid hydrolysis and a second compartment for methane fermentation) separated by a partition wall. Both compartments are in fluid communication by a channel which is permanently open.
FR-A-2 510 605 discloses a bioreactor for production of methane from organic waste by anaerobic fermentation, which comprises two compartments that are separated by a partition wall. Although both compartments are permanently in liquid communication with each other, the backflow of liquids is prevented by a series of chains disposed in the channel that connects both compartments.
FR-A-2 305 113 discloses a bioreactor for anaerobic fermentation, which comprises two compartments that are separated by a partition wall. Although both compartments are permanently in liquid communication with each other, the backflow of liquids is prevented by a particular shape of the partition wall.

DE-A-197 15 646 discloses a bioreactor for production of methane from organic waste by anaerobic fermentation in which the fermentation tank is not partitioned.
These fermentation reactors in the prior art have their respective problems and disadvantages. In the methane fermentation reactors having a built-in gas holder, for example, it is sometimes the case that backflow of the liquid, i.e. digestion liquid, contained therein takes place from the methane fermentation tank to the acid fermentation tank or to the raw waste receiver tank or from the settling tank to the methane fermentation tank as a result of the variation in the internal pressure of the gas holder. Since the methane fermentation can proceed by the growth of an absolutely anaerobic microorganism which necessitates an oxidation-reduction potential of 150 mV to 400 mV, the backflow of the liquid from the methane fermentation tank to the acid fermentation tank naturally leads to a decrease in the methanogen concentration in the methane fermentation tank resulting in a decrease in the efficiency of methane fermentation. The backflow of the liquid from the settling tank to the methane fermentation tank means inflow of the dissolved oxygen contained in the liquid in the settling tank into the methane fermentation tank resulting in a partial decrease of the activity or dissolution of the methanogen microorganisms. Thus, backflow of liquids between the component tanks causes a great decrease in the methane fermentation capacity of the reactor and instabilization of the process if not to mention the problem that the reactor cannot be compact enough because the volume of the fermentation tanks must be so large as to comply with the backflow of the liquid.
The above described disadvantages provide inhibition against use of the methane fermentation reactors having a built-in gas holder so that the reactors of this type have no noticeable prevalence despite the advantageously low costs.
The present invention accordingly has an object to provide, by overcoming the above described problems and disadvantages in the conventional methane fermentation reactors of the built-in gas holder type, a high efficiency methane fermentation reactor capable of being operated over a long period of time by keeping high capacity with stability.
The inventor has conducted extensive investigations with an object to develop a high-efficiency two-phase type methane fermentation reactor and arrived at an unexpected discovery that, when gate valves are provided between the acid fermentation tank and the methane fermentation tank and between the methane fermentation tank and the settling tank, of which the gate valves allow one-way flow of the liquid from the acid fermentation tank to the methane fermentation tank and from the methane fermentation tank to the settling tank, and the gate valves are controlled by utilizing the up-and-down movement of the liquid level and pressure changes in the gas holder in such a way that the feed liquid flows and is discharged successively through the acid fermentation tank, methane fermentation tank and settling tank, backflow of the liquid between the respective tanks can be efficiently prevented leading to completion of the present invention on the base of this discovery.
Thus, the two-phase methane fermentation reactor provided by the present invention is an integral system which comprises:
(a) an acid fermentation tank which receives a feed liquid for fermentation treatment;
(b) a methane fermentation tank of a gas holder type positioned side-by-side to the acid fermentation tank, of which the upper space serves as the methane gas holder;
(c) a first vertical partition wall which partitions the acid fermentation tank and the methane fermentation tank;
(d) a settling tank positioned side-by-side to the methane fermentation tank, out of which the digestion liquid is discharged;
(e) a second vertical partition wall which partitions the methane fermentation tank and the settling tank;
(f) a first gate valve provided on the first partition wall for controlling the liquid flow between the acid fermentation tank and the methane fermentation tank;
(g) a second gate valve provided on the second partition wall for controlling the liquid flow between the methane fermentation tank and the settling tank; and
(h) a methane gas outlet tube having a stopcock valve connected to the methane gas holder.
It is optional but advantageous that the above defined reactor of the invention is constructed as a semi-underground installation or full underground installation so that an improvement can be obtained for the heat insulation of the fermentation tanks. If necessary for further improvement of the efficiency, the temperature of the feed liquid introduced into the acid fermentation tank is moderately increased to 20 to 30xc2x0 C. by means of, for example, supplying air to the feed liquid receiver tank so as to cause aerobic decomposition of the waste material with generation of heat.